Recently, liquid crystal display cells excellent in high-speed response, wherein ferroelectric liquid crystals proposed by N. A. Clark et al. are used, have been applied to large-sized liquid crystal displays. The light switching of a chiral smectic C phase (which will be called an "S.sub.C * phase" hereinafter) is applied to these display cells. A chiral smectic liquid crystal mixture obtained by adding a chiral compound to a liquid crystal mixture showing a smectic C phase (which will be called an "S.sub.C phase" hereinafter) is effective as the above-mentioned liquid crystal material showing an S.sub.C * phase.
In order to secure a wide operating temperature range of a display cell of the above-mentioned type, it is required that the chiral smectic liquid crystal mixture shows the S.sub.C * phase over a wide range of temperature.
In the preparation of the chiral smectic C liquid crystal mixture showing an S.sub.C * phase over a wide range of temperature, it is effective to use a smectic C liquid crystal mixture, to which a chiral compound is to be added, showing an S.sub.C phase over a wide range of temperature.
A common method for the preparation of a smectic liquid crystal mixture showing an S.sub.C phase over a wide range of temperature comprises mixing smectic liquid crystals showing an S.sub.C phase within a low temperature range with those showing an S.sub.C phase within a high temperature range. Thus there has been desired to provide various smectic liquid crystal compounds satisfying the following three requirements.
(1) It shows an S.sub.C phase over a wide range of temperature.
(2) It shows no higher-order smectic phase than that of the S.sub.C phase.
(3) It lowers the melting point, when mixed with a smectic liquid crystal mixture showing an S.sub.C phase.
It has been further desired to provide various smectic liquid crystal compounds which can enlarge the S.sub.C phase temperature range of the resulting composition, when added to a smectic liquid crystal mixture showing an S.sub.C phase.